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Incipient motion of microplastic particles: conformity with the Shields diagram under its original experimental premises
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Researchers tested 3D-printed plastic particles of various sizes and densities in a rotating water flume, recreating the exact conditions of a classic 1930s experiment on when particles start moving in flowing water. The plastic particles behaved just like the original non-plastic particles, suggesting that earlier studies finding plastics behave differently were likely affected by their experimental setups rather than plastic itself.
Abstract Predicting the transport and fate of microplastics (MPs) in aquatic environments requires a reliable understanding of the incipient motion of deposited particles. While several previous studies have attempted to apply the Shields diagram to MPs, they have commonly reported that low-density MPs deviate from the threshold curve and have proposed various correction frameworks. However, it remains unclear whether these deviations stem from the intrinsic material properties of plastics or from experimental conditions that depart from the fundamental premises underlying the Shields diagram. To rigorously evaluate the applicability of the Shields diagram to plastic materials, this study reproduced the original experimental conditions of Shields (i.e., a flat, uniform, granular bed composed of spherical particles of identical material) using 3D-printed plastic particles tested in an annular flume. Experiments were conducted on particles with densities ranging from 1.05 to 1.31 g/cm³ and diameters from 1 to 20 mm, spanning both micro- and macroplastic size ranges. The results show that all plastic data are distributed in close proximity to the Shields diagram, demonstrating that the observed deviations in prior studies are attributable not to the material properties of plastics, but to the complex, applied experimental conditions employed in those studies.
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